1530s, "state or condition of being smaller," a sense now obsolete, from Middle French minorité (15c.), or directly from Medieval Latin minoritatem (nominative minoritas), from Latin minor "less, lesser, smaller, junior" (see minor (adj.)).
Meaning "state of being under legal age" is from 1540s; that of "smaller number or part, smaller of two aggregates into which a whole is divided numerically" is from 1736. Specifically as "the smaller division of any whole number of persons" (in politics, etc.) is by 1789. The meaning "group of people separated from the rest of a community by race, religion, language, etc." is from 1919, originally in an Eastern European context.